Breeding objectives and economic values in two different beef cattle production systems
Beef cattle production systems can be more efficient with the correct establishment of breeding objectivs and the use of economic values. The aim of this study was to define breeding objectives and to derive economic weights for traits considered economically important in two production systems of crossbred Angus x Nelore. Two bioeconomic models were developed in Excel Software 2010 by simulation, using biological, economic and performance data of beef cattle production systems with cow-calf cycle (CC) and complete breeding cycle (CB). Weaning weight (WW) in kg, and weaning rate (WR) in % were chosen as breeding objectives for CC, in addition to traits year weight (YW), weight at 18 months (W18), and slaughter weight (SW) in kg, for CB. The impact of the traits on the profit of the production system was assessed by the increase of 1% in the performance of each trait and others remaining unchanged. The economic values were obtained by dividing marginal profit by genetic gain (EV=MP/GG), where MP was obtained by the difference between profit before and after the improvement of the traits, and GG was obtained by the difference between performance of the trait before and after the improvement (based on cow/year). The calculations were based in Brazillian currency (R$) and converted to American currency (US$) using the average exchange rate in 2015 ($1.00 = R$3.33). Positive changes were found in profit after selection in both systems. In CC, each 1% increase in the WW and WR resulted in increase of $1.75 and $4.98 per cow/year in profit, respectively. The CB system obtained for WW, WR, YW, W18, and SW, respectively, the economic values of $1.03, $6.48, $1.82, $1.22 and $1.36 per cow/year. The bioeconomic model developed was appropriate for the proposed objectives, in the production systems of this study. WR showed the greatest impact on the profit in both production systems. This trait is important for determining the number of animals which will be marketed. The largest gains for the CB system can be obtained by selection for WR, followed by YW, SW, W18 and finally, by WW. Additional studies should be conducted about inclusion of these traits in development a multiple trait selection index.